The hero of The King Must Die, Theseus is brave and proud. He believes that he is the son of the god Poseidon, and he is determined to make himself worthy of his father. Theseus achieves much at such a young age because he seeks challenges. Theseus is eighteen years old when he kills Asterion and makes his way home from Crete, but his deeds are those of an accomplished warrior. He is king of Eleusis at age seventeen. Theseus does not fear his death, because he believes that he hears the voice of Poseidon. When he feels that he is working towards justice and his actions are right, there is no reason for him to worry about the consequences. For Theseus, dishonoring himself or the gods would be worse than death. When he dies, he goes to the gods with nothing but a record of his life, and if he lived honorably then he has lived a good life. But he holds his belief in the gods above even his honor. As a king, he acts as an intermediary between his people and the gods. His sense of responsibility for his people allows Theseus to keep his cool when Asterion insults him. Although he desperately wants to kill the Minotauros, he knows that the lives of the Cranes and many others depend upon him, and if he gives in to his urge for vengeance it will hurt the people. Theseus is also clever, and he often uses his wits more than his strength. His agility and quickness of mind make him a great warrior. His keen intelligence makes him a good king, because he understands how to rule and how to deal with people. Because he treats people well and looks out for their lives, Theseus inspires great loyalty.
Asterion is Theseus' nemesis. Theseus wishes to be a King because he believes it is his birthright. Asterion wants to be King because he craves power for its own sake. He is very intelligent and cunning. Unlike Theseus, Asterion enjoys using his power. He makes all of the Cretan nobles fear and disrespect him. Asterion treats Theseus like an animal since he is a slave. Asterion believes that he can use people like puppets, but he forgets that they have their own free will. Many of the Cretan nobles have sacrificed their freedom because of their fear of him, but Theseus refuses to submit to Asterion. Asterion is cruel, and just as he cares for no one but himself he believes in nothing beyond himself. Asterion tries to use the gods as puppets. In the same way that he underestimates the potential of people to act of their own accord, he also forgets the great role that chance plays in life. Whether actions are attributed to the gods or to nature, what is clear is that the unexpected happens. Asterion tries to rule using his tremendous wealth and power to inspire fear in others, but his power base is transient, and, unlike Theseus, no one is loyal to him. Those who support Asterion do so because they are coerced.
Aigeus is Theseus' father, and, although he is a good ruler, he lacks Theseus' passion for life. The stress caused by years of ruling under the constant threat of war has worn him down. Aigeus is a good man who cares deeply for his son and when Theseus arrives in Athens it reinvigorates him. After Theseus volunteers to go to Crete he is crushed, and from Theseus' perspective he is not such an admirable character, but in reality, he is simply a father reacting to the possibility that he will lose his son. Moreover, the older Theseus who tells the tale seems to be closer to Aigeus then to the younger Theseus the story describes. By the time he reaches old age, Theseus has seen many hardships and much toil. Aigeus' character may represent the result of trying to do the best for the people. It is not always possible to achieve positive outcomes, ruling sometimes requires choosing the lesser of two evils. Aigeus, is a man who has tried his entire life to succeed at a job that everyone eventually fails at. His attempt is noble and instructive. Theseus, however, believes he will do better.